We propose a study of cholesterol metabolism and the effects of cholesterol deficiency in Smith-Lemli-Opitz Syndrome (SLOS). SLOS is a disorder of cholesterol synthesis caused by mutations in the DHCR7 gene encoding 7-dehydrocholesterol (7DHC) reductase, the final enzyme in the cholesterol synthetic pathway. Affected individuals exhibit multiple malformations and mental retardation. The features of SLOS are thought to be primarily related to cholesterol deficiency and accumulation of 7DHC. However, the clinical phenotype is not well characterized, the biochemical pathogenesis is incompletely understood, and there is no proven therapy for this devastating condition. Thus our first objective is to better define the phenotype of SLOS using a natural history study design. We hypothesize that impaired cholesterol homeostasis leads to measurable behavioral and neurocognitive deficits, impaired brain myelination and cholesterol turnover, and retinal dysfunction. To test this hypothesis we will assess cholesterol homeostasis using state-of-the-art methods in parallel with clinical observation, testing, and imaging. This natural history sub-study will contribute to creating a comprehensive SLOS natural history registry and to the development of end-points for clinical trials. Our second objective is to test the efficacy of simvastatin as a complementary therapeutic strategy in patients supplemented with cholesterol. We hypothesize that SLOS patients will respond favorably to simvastatin treatment by improving brain cholesterol synthesis and increasing whole body cholesterol pool size. To test this hypothesis, we will treat SLOS patients supplemented with cholesterol for 2 years with simvastatin. Treatment efficacy will be judged primarily on changes in cognition and behavior (clinical) but also on surrogate biochemical and other measures (i.e. sterols and oxysterols, ERG, and brain MRI), in comparison with patients receiving only cholesterol supplementation. This intervention study will test the feasibility of clinical treatment trials in SLOS and the likelihood of efficacy of a promising intervention, as well as provide a foundation for future multicenter clinical trials. In this project, we plan to proceed with translation of in vitro studies from bench to bedside. Our third objective is to elucidate SLOS pathogenesis, probe the consequences of DCHR7 deficiency on cell functions and evaluate the cellular benefit of compounds with therapeutic potential in vitro. We hypothesize that DCHR7 deficiency causes metabolic diversion away from cholesterol synthesis, alters the structure, composition and signaling function of plasma membrane caveolae, impairs ER-specific protein folding activity, and causes cellular oxidative stress and apoptosis. We further hypothesize that statins, bile acids, antioxidants and molecular chaperones selectively restore SLOS cell metabolism and function. These latter studies will be conducted in vitro using SLOS and control skin fibroblasts, SLOS mouse brain-derived cells, and SLOS human brain tissues. Together, the in vivo and in vitro studies proposed should shed light on the pathogenesis of SLOS, and offer insights into treatment that to date have eluded investigators.